Design Elements for Enhanced Hydrogen Isotope Separations in Barely Porous Organic Cages

Barely porous organic cages (POCs) successfully separate hydrogen isotopes (H-2/D-2) at temperatures below 100 K. Identifying the mechanisms that control the separation process is key to the design of next-generation hydrogen separation materials. Here, ab initio molecular dynamics (AIMD) simulations are used to elucidate the mechanisms that control D-2 and H-2 separation in barely POCs with varying functionalization. The temperature and pore size dependence were identified, including the selective capture of D-2 in three different CC3 structures (RCC3, CC3-S, and 6ET-RCC3). The temperature versus capture trend was reversed for the 6ET-RCC3 structure, identifying that the D-2 and H2 escape mechanisms are unique in highly functionalized systems. Analysis of calculated isotope velocities identified effective pore sizes that extend beyond the pore opening distances, resulting in increased capture in minimally functionalized CC3-S and RCC3. In a highly functionalized POC, 6ET-RCC3, higher velocities of the H isotopes were calculated moving through the restricted pore compared to the rest of the system, identifying a unique molecular behavior in the barely nanoporous pore openings. By using AIMD, mechanisms of H-2 and D-2 separation were identified, allowing for the targeted design of future novel materials for hydrogen isotope separation.